There are a number of important antecedents to this invention. The most important antecedents are Stirling engines and refrigerators, a century old. An important step in the elimination of moving parts from Stirling engines and refrigerators came in 1969, when William Beale invented the "free-piston" variety of Stirling devices, in which the crankshaft and linkages were replaced by gas springs, so that gas spring constants and piston masses could be chosen to cause resonant motion of the pistons with the desired frequency, amplitudes, and phases.
Ceperley, "Gain and efficiency of a short traveling-wave heat engine," 77 J. Acoust Soc. Am., pp. 1239-1294 (1985) suggested that the essence of Stirling engines and refrigerators is a regenerator (and adjacent heat exchangers) in which the pressure and velocity oscillations are substantially in phase, reminiscent of an acoustic traveling-wave, and hence that an acoustic network with essentially toroidal topology containing the Stirling heat-exchange components can provide such phasing. Ceperley claimed that efficiencies near 80% of the Carnot efficiency are in principle possible with such configurations. Ceperley's contribution could be seen as an extension of Beale's, in that Ceperley uses gas inertia effects in addition to Beale's gas spring effects, thereby eliminating the massive pistons of Beale's invention. Other related teachings by Ceperley are set out in U.S. Pat. No. 4,113,380, issued Sep. 19, 1978, and U.S. Pat. No. 4,355,517, issued Oct. 26, 1982. However, Ceperley presented no teachings on how to realize a practical device.
The conventional orifice pulse tube refrigerator (OPTR) (Radebaugh, "A review of pulse tube refrigeration," 35 Adv. Cryogenic Eng., pp. 843-844 (1992)) operates thermodynamically like a Stirling refrigerator, but with the cold moving parts replaced by passive components: a thermal buffer column known as the pulse tube, and a dissipative acoustic impedance network. The efficiency Q.sub.C .vertline.W of an OPTR is fundamentally limited by the temperature ratio T.sub.C /T.sub.0, which is lower than the Carnot value T.sub.C /(T.sub.0 -T.sub.C) because of the inherent irreversibility in the dissipative acoustic impedance network. T is temperature, Q.sub.C is heat, W is work, and the subscripts 0, and C refer to ambient and cold, respectively. The OPTR can be regarded as another means to eliminate moving parts from Stirling devices. However, the efficiency of an OPTR is fundamentally less than that of a Stirling device, and the OPTR is only applicable to refrigerators.
Conventional OPTRs have long used the thermal buffer column known as a pulse tube, but until recently this component carried substantial heat leak. However, using a tapered tube, as described in U.S. patent application Ser. No. 08/975,766, filed Nov. 21, 1997, can reduce the heat leak along such a thermal buffer column to as little as 5% of the cooling power of a OPTR. Thermal buffer columns have been used in two-piston Stirling refrigerators as well as in OPTRs, but not in Stirling engines.
In the context of double-inlet OPTRs, Gedeon, "DC gas flows in Stirling and pulse-tube cryocoolers," in Ross ed., Cryocoolers 9, pp. 385-392 (Plenum, N.Y. 1997) discusses how nonzero time-averaged mass flux M can arise in Stirling and pulse-tube cryocoolers whenever a closed-loop path exists for steady mass flux. It is essential that M through a Stirling engine or refrigerator be near zero, to prevent a large steady energy flux Mc.sub.p (T.sub.0 -T.sub.C) from adding an unwanted thermal load to the cold heat exchanger of a refrigerator, or to prevent a large steady energy flux Mc.sub.p (T.sub.H -T.sub.0) from removing a large amount of heat from the hot heat exchanger of an engine--in either case, reducing the efficiency. Here c.sub.p is the gas isobaric specific heat per unit mass.
Another, less directly related antecedent to this invention is the set of prior thermoacoustic engines and refrigerators developed in the past 20 years at Los Alamos National Laboratory and elsewhere. These operate on an intrinsically irreversible cycle, using nearly standing-wave phasing between gas pressure oscillations and velocity oscillations and using deliberately imperfect thermal contact in the stack (which might otherwise be mistaken for a regenerator). The intrinsic irreversibility and other practical issues have thus far limited the best standing-wave thermoacoustic engines and refrigerators to below 25% of the Carnot efficiency.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.